Half of Greenland's Warming Tied to Natural Causes

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About half of the surface warming that's helping shrink
Greenland's glaciers is due to temperatures in the tropical
Pacific Ocean, not greenhouse gases, a new study reports.

Sea-surface temperatures in the Pacific are already known to
influence global weather patterns at lower latitudes. For
example, the El Niño cycle shifts rainfall around the world,
delivering precipitation to western North America and causing
drought in Australia and Central America.

The new findings could explain why Greenland and the
Canadian Arctic are getting hotter more quickly than
other regions of the planet. The feverish temperature rise has
puzzled scientists: The most up-to-date climate models, such as
those in the fifth assessment report of the Intergovernmental
Panel on Climate Change, fail to reproduce the rapid warming seen
in the Arctic. [ On
Ice: Stunning Images of Canadian Arctic ]

"We know that global warming due to human impacts can't explain
why it got warm so fast," said lead study author Qinghua Ding, a
climate scientist at the University of Washington.

Researchers have proposed several explanations for the speedy
heating, such as a warmer Arctic Ocean from sea ice loss.

But Ding and his co-authors instead see a link between tropical
sea-surface temperatures and the North Atlantic Oscillation, a
climate pattern that dominates Arctic weather. Since the 1990s,
warm sea-surface temperatures in the western Pacific and cool
waters in the eastern Pacific have pushed the North
Atlantic Oscillation (NAO) into a pattern that allows
high pressure above Greenland and the Canadian Arctic. (High
atmospheric pressure leads to warmer temperatures.)

"We find that 20 to 50 percent of the warming is due to
anthropogenic [man-made] warming, and another 50 percent is
natural," Ding said. The study was published today (May 7) in the
journal Nature.

The NAO is a major climate player, as it affects the extent of
Arctic sea ice; the path of the jet stream; and storm routes
across North America, the Atlantic and Europe. Finding a
connection between the NAO and the tropics could improve
forecasts for the NAO, which has defied accurate prediction.

"The common sense was that the NAO is chaotic, not connected to
tropical ocean conditions," said Shang-Ping Xie, a climate
scientist at the Scripps Institute of Oceanography, who was not
involved in the study. "An obvious implication is that this
connection may be exploited to improve climate prediction over
the extratropical North Atlantic where the current prediction
skills are low."

The connection between the Pacific Ocean and Greenland comes from
atmospheric pulses called Rossby
waves. These are undulations in the high-altitude winds
that race around the globe, such as the jet stream. The
distribution of warm and cold air rising above the Pacific Ocean
sets off a Rossby wave that eventually favors warmth over
Greenland.

"It's like hitting the atmosphere with a hammer in a very
specific region, which generates a wave train that causes high
pressure over Greenland," Ding said.

New connections to explore

Tropical ocean temperatures have only been closely watched since
1979, with the advent of satellites, so the researchers don't
know if the Pacific temperature cycle is short lived or if it has
settled in for decades.

However, the Pacific Ocean warmth is not the same as the El Niño
cycle, Ding said. The researchers plan to explore whether the
ocean temperatures could be linked to other known climate cycles,
such as the Pacific
Decadal Oscillation, or if this is a newly discovered
variation.

"This study shows how complex regional climate change is," said
Juergen Bader, a climate scientist at the Max Planck Institute
for Meteorology in Germany, who was not involved in the study.
"Even remote processes can have an important impact on the
regional climate." [ 6
Unexpected Effects of Climate Change ]

If the Pacific temperature pattern shifts, warming in the Arctic
could slow in coming decades, Ding said. Some evidence already
hints this is the case, such as jet stream pattern that socked
the East Coast with an extremely cold winter this year. However,
human-driven global
warming is likely to outpace any natural cooling in
coming decades, researchers said.

"It is only a question of time before external forcing [man-made
warming] dominates regional Arctic warming," Bader said. "So the
role of natural climate variability on certain Arctic warming
patterns might be reduced in the long run."